In recent years, necessity of global environment conservation is noised abroad, and a cogeneration apparatus serving as a privately-owned electrical power facility which performs electric power generation, hot-water supply, and the like by using an engine such as a gas engine using city gas or the like as fuel as a power source attracts attention. In the cogeneration apparatus of this type, a heat output generated by electric power generation cannot be simultaneously consumed in many cases. From the viewpoint that energy is used without waste, a heat-demand-priority type apparatus which does not generate an amount of heat which cannot be consumed is proposed. For example, a cogeneration apparatus described in Japanese Patent Application Laid-open No. 2000-87801 is operated only when heat request is occurred from a heat load side to improve operating efficiency. In Japanese Patent Application Laid-open No. 2000-297963, a heat-demand-priority type apparatus which further includes a hot-water supply tank as a buffer for heat output and heat demand and which temporarily stores an amount of heat as hot water in the hot-water supply tank when the heat output and the heat demand are not equal to each other. According to this apparatus, an amount of heat can be used through hot water depending on heat demand in order to make it possible to reduce waste when the cogeneration apparatus does not operate.    Patent Document 1: JP 2000-087801 A    Patent Document 2: JP 2000-297963 A